The present invention generally relates to a semiconductor package and more particularly to a semiconductor package having a bridged plate connecting a power semiconductor device source metalized contact and a leadframe source lead.
Semiconductor devices are conventionally connected to leadframe leads using either plate interconnections or wire bonding. For example, U.S. Pat. No. 5,821,611 discloses a semiconductor device which comprises a first lead having a tip formed with an island, a semiconductor chip unit mounted on the island of the first lead by means of a solder layer and having a plurality of electrode bumps projecting away from the island, and a plurality of additional leads each of which has a tip electrically connected to the electrode bumps via respective solder deposits. The additional leads include at least second and third leads. The leads are alloyed to the electrode bumps in a heating furnace and the solder bumps may spread during heating and create undesirable shapes.
U.S. Pat. No. 6,040,626 discloses a semiconductor package which employs a mixed connection between a MOSFET top surface comprising a low resistance plate portion for connecting to a source and a wire bond for connecting to a gate. Wire bonding may introduce short circuits in the device due to device dielectric layer damage during the wire bonding process.
A semiconductor package with directly connected leads is disclosed in U.S. Pat. No. 6,249,041. A semiconductor device includes a semiconductor chip with contact areas on the top or bottom surface. A first lead assembly, formed from a semi-rigid sheet of conductive material, has a lead assembly contact attached to one of the contact areas of the semiconductor chip. The first lead assembly also has at least one lead connected to and extending from the lead assembly contact. A second lead assembly, also formed from a semi-rigid sheet of conductive material, has a lead assembly contact attached to another one of the contact areas of the semiconductor chip. The second lead assembly also has at least one lead connected to and extending from the lead assembly contact. An encapsulant encloses the semiconductor chip, the lead assembly contact of the first lead assembly and the lead assembly contact of the second lead assembly. The semiconductor device has low electrical and thermal resistance contributions from the package due to the direct connection of the lead assemblies to the chip. The lead assembly contact areas are held in contact with lead contact areas on the semiconductor chip by an electrically conductive adhesive layer. The electrically conductive adhesive layer may be a silver-filled epoxy or polyimide paste or solder bumps. The adhesive layer may be cured in a curing oven, if necessary. The adhesive layer does not include soft solder or solder paste.
Another semiconductor package with directly connected leads is disclosed in U.S. Pat. No. 6,479,888. A MOSFET comprises a plurality of inner leads electrically connected to a surface electrode of a semiconductor pellet having a field effect transistor on a principal surface thereof. The inner leads are mechanically and electrically connected to the principal surface by a gate connecting portion and source connecting portions constituted by bumps.
A common problem encountered in the use of patterned plate or clip interconnections is that the plates or clips may float during solder reflow and result in a misaligned interconnection. In some cases, the misalignment results in a short circuit between the source and gate contact areas resulting in low assembly yields. Furthermore, it is difficult to control the required solder volume to prevent this problem.
Another problem resulting from the use of patterned plate or clip interconnections is the thermal expansion mismatch between the silicon of the semiconductor device and the metal of the patterned plate or clip. The larger the contact area of the patterned plate or clip, the higher the stress induced by the mismatch, which often results in die cracking. To reduce the stress, a smaller plate or clip may be used. However, the smaller contact area may result in higher resistance.
There is therefore a need in the art for a semiconductor package that includes a semiconductor power device connected to leadframe source and gate leadframe contact areas by means of patterned plates that overcomes the problems of the prior art. There is also a need for a patterned plate interconnection that does not float during solder reflow and ensures precise clip placement and location assurance. There is also a need for a semiconductor package having device metallized areas for restricting the flow of solder during the soldering process. There is also a need for a metalized area formed of Ni/Au. There is also a need for a semiconductor package process that increases throughput and provides easier assembly process control. There is also a need for a semiconductor package method that provides a soft attachment process of the patterned plates onto the semiconductor power device. There is also a need for a semiconductor package having an exposed source plate. There is also a need for a semiconductor package having reduced electrical resistance. There is a further need for a semiconductor package having improved thermal dissipation properties. There is also a need for a semiconductor package having improved mechanical properties.